(1) Field of the Invention
This invention relates to a spark plug for internal combustion engines, and more particularly it is concerned with a spark plug including electrodes of increased resistance to wear.
(2) Description of the Prior Art
In one type of spark plug known in the art, a center electrode of the spark plug has joined thereto by welding a metal tip formed of precious metal, such as platinum, or other metal of high resistance to heat and wear.
The ignition section of a spark plug is exposed to combustion gas and hence is sensitive to changes in temperature caused by various driving conditions of an internal combustion engine. The temperature is greatly governed by driving conditions and it is relatively low when the engine is driven for rotation under low load but rises to a markedly high level when the engine is driven for rotation at high speed under high load. Thus the spark plug of the aforesaid type of the prior art has had the disadvantage that due to differences in the coefficient of thermal expansion, thermal stresses develop in the center electrode and the metal tip joined thereto by welding. The thermal stresses are in proportion to temperature, and act repeatedly under conditions of repeatedly rising and falling temperature. The thermal stresses apply an excessively high load to the metal tip, causing rupture of the metal tip to occur.